1. Field of the Invention
The present invention relates generally to wafer processing, and in particular to methods of processing an oxygen-containing wafer using rapid laser annealing (RLA) to form a denuded zone at the wafer surface.
2. Description of the Prior Art
Single crystal silicon wafers for semiconductor manufacture are usually cut from large single crystals grown from liquid silicon using a process called the “Czochralski” process. In this process, molten silicon is contained in a vessel made from silicon oxide (SiO2) so that the resulting silicon crystal contains about 1018 atoms of oxygen per cubic centimeter. Depending on the thermal history of the silicon wafer, the oxygen atoms occupy substitutional sites in the crystal lattice.
During crystal growth, vacancies and interstitials are both present in the ingot. Which of these defects dominate depends on the ratio of crystal growth (v) to the axial thermal gradient at the interface (G). A high v/G ratio results in a vacancy-dominant crystal, while a low v/G ratio results in an interstitial-dominant crystal. Once the crystal cools down, the point defects agglomerate into larger defects. The interstitials form 1 to 2 mm dislocation loops (known as “A type” dislocations). The vacancies form 50-200 nm voids known as COPs D-defects (or just “COP defects” for short), where COP stands for “crystal-oriented particle”.
The most economical way to grow a silicon crystal is as fast as possible. However, fast crystal grown results in a vacancy-dominant structure that leads to COP defects in the crystal. The COP defects present on the surface of the wafer can be problematic. In particular, a COP defect can adversely affect the gate dielectric quality, even to the point in extreme cases where the gate is totally shorted. One of the benefits of high-temperature annealing is that it reduces the amount of COP defects at the same time it generates oxygen precipitates that act as gettering centers for process impurities that otherwise will interfere with the device operation.
The electrical circuitry eventually formed in and on the wafer during the semiconductor manufacturing process occupies only a very thin section of the wafer body immediately adjacent the wafer surface. Accordingly, one approach to avoiding the hazards presented by the aforementioned oxygen-related surface defects is to grow an epitaxial layer of pure silicon on the wafer surface. This can be done using, for example, chemical vapor deposition (CVD). The epitaxial layer is usually lightly doped as compared to the underlying single-crystal wafer. The underlying single crystal wafer serves as a template that orients the atoms so that the epitaxial layer is atomically aligned to the underlying single crystal. The vapor-phase grown epitaxial layer is relatively free of oxygen atoms so that the above-described problems with oxygen-related surface defects is avoided. Unfortunately, growing an epitaxial layer on a silicon wafer is an expensive proposition, adding about a 20-50% premium to the cost of an ordinary silicon wafer.
Another approach to reducing oxygen-related surface defects that is less expensive than growing an epitaxial layer is to reduce the concentration of oxygen atoms near the wafer surface by heating the wafer in an inert atmosphere. This allows the oxygen near the wafer surface to out-diffuse and form an oxygen-depleted surface region called a “denuded zone”. Wafers processed in this manner are commercially available from a variety of wafer manufacturers. The typical approach is to use a furnace to heat the entire wafer to a temperature between 600° C. and 700° C. for one to two hours, and then to rapidly cool the wafer. However, this is a time-consuming process and the rapid cooling is hard to carry out in this furnace-based approach.
Yet another approach to forming a denuded zone in an oxygen-containing wafer involves flash annealing. Flash annealing uses millisecond-long, high-intensity flashes from a flash lamp to irradiate the entire top wafer surface at once. However, the sudden burst of energy heats only the very top surface of the wafer and causes the entire wafer to bow severely. The resulting stresses may result in a permanently warped wafer if the stress levels exceed the elastic limit for the wafer material at the maximum temperature. Unfortunately, the available time durations of the pulse and the peak temperatures that are accessible with flash annealing cannot be used to create an optimal denuded zone.
What is needed is a fast and efficient approach to form a denuded zone at the surface of an oxygen-containing semiconductor wafer that overcomes the above limitations.